I have done some body work but I have much to learn. I tried to remove some dents from a pannel and I ended up stretching the area some. Is the general rule that you will always have to shrink a area that you worked a dent out of and If you used (hammer on dolly) you will have to shrink the area a lot?

On one of the Spike Sunday morning how to do it shows they talked and showed how on this same subject. I think there are other shows out there also that have done the same thing. There are many tools for this besides heating the area including special metal shrinking hammers. I just did a quick search and there are numerous videos on the subject on line.

As I remember you have to plan ahead on working metal so any shrinkage is minimal. To watch those who know how to do it they seem to get the metal to do it itself (finessing the metal) rather then beating it into shape. Remember too much working of the metal can work harden it which is another problem. http://forum.eastwood.com/showthread.ph ... heet-Metal

If the dent has a stretched spot , like when the impact slides across the surface ,it is harder to repair it than a ding. I am old school , my old Ghia roof looked like 14 storm troopers walked across the roof when I got it. When I bumped out the roof I finished it with a body metal file , used no body filler . Once the metal is stretched it requires spending a lot more time to repair , patience is key .

I was doing some work on a bottom door panel that needed the door frame patched, the part where the lower door skin attatches to. The lower part of the door frame was holding the contour of the upper door so when it rusted out the door bowed in ,instead of out. I got the door shin to bow out but I put some small dents in the door in the process. I worked these out the best I could and I had a bit of a dome left in the countour afterwards. I was thinking that when a piece of metal is bent like in angle iron, it stretches the outside and 'sort of ' shrinks the outside. When the angle is bent out of the piece of metal then the opposite happens leaving stretched metal on both sides. So I'm thinking that the metal is ALLWAYS stretched to some degree when bent.

Also note there is a soft supposedly more "workable" MIG wire available from ESAB, MIG welds are usually hard as rocks due to the rapid cooling and typical carbon content, MIG welds harden from rapid cooling as well s work hardening.

Shrinking discs don't put enough heat into the metal to re-anneal (friction), but do put enough in to shrink with the immediately following damp wipe down pre-annealing the area if it has been work hardened may help.

Of course 304L SS TIG rod is pretty soft and does not harden from the weld, but will work harden.
A torch can be used to re anneal the area to allow further work.

I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

If you have metal with a bulge or crown in it it could be that both sides can be stretched but one side is stretched more than the other. If the bottom is pulled out then the inside of the material can be stretched or the inside is either shrunk or stretched less.

If both sides were stretched evenly the material would be longer. Look for metal thinning also.

(for what it is worth. Just some additional information) One thing you have to worry (probably not with what we are normally doing) about with metal stretching and it is fairly common with deep draw metal forming and is usually shows up in spherical radiuses is metal tearing. Lets assue that you are forming a box with the sides and the bottom being formed from the same flat sheet. With a radius in the bottom and sides and the formed sides where they joined, at the point where the radiuses meet there would be a formed "spherical Radius"; that is where the metal is stretched in several directions. We were OK with a 3:1 ratio deep draw but then starting at ~6:1 or deeper of a deep draw (this is AL I am now talking about), that would be where the tearing could be found quite often. Basically we were stretching both the inside and outside of the radius with the outside being stretched more.